1. Field of the Invention
The present invention relates to a magnetron sputtering apparatus for depositing a thin film on, for example, a semiconductor wafer and to a sputtering gun used in the magnetron sputtering apparatus.
2. Description of the Related Art
A magnetron sputtering apparatus has a vacuum chamber. The vacuum chamber accommodates a substrate (e.g. a semiconductor wafer) on which a thin film is to be formed, and a sputtering gun. The sputtering gun has a target situated to face the substrate, and a sputtering electrode.
In sputtering, a sputtering gas atmosphere (generally, argon gas) of an m Torr order is maintained in the vacuum chamber, and a negative voltage is generally applied from a DC power supply to a sputtering target via a sputtering electrode having a cooling mechanism. On the other hand, such parts as an anode, a shield, etc. which are situated at a distance of several millimeters from the sputtering target and sputtering electrode are generally kept at a grounding potential along with the vacuum chamber. A magnetic circuit is provided near the rear surface of the sputtering target. Thereby, a parallel magnetic field is applied to the surface of the sputtering target. Thus, a crossed electromagnetic field is produced on the surface of the sputtering target and thereby a cyclonic motion of electrons occurs in the plasma. Stable sputtering can be performed with a relatively low pressure.
Generally, a target of a conventional sputtering gun has a flat shape or an inverted-conical sputtering surface having a gentle gradient, and the diameter of the target is greater than that of a wafer.
With a recent increase in IC integration density, the diameter of a via hole or a contact hole is decreased to, e.g. 0.5 micron while the thickness of the wafer is substantially unchanged. As a result, the aspect ratio of the via hole or contact hole increases.
When a thin film is formed on such a high-integration wafer with use of a conventional flat target, the following problems arise.
FIG. 1 shows a sputtering apparatus having a flat target 2. Sputter particles emitted from the respective areas of the surface of the target 2 include various directional components, as shown in circles 3.
A ULSI 1 of high integration of 4M-DRAM or above, which is used as substrate, has a contact hole (or via hole) 5 having a diameter of 0.5 micron and a depth of 1 micron (i.e. aspect ratio=2). When a conductive film 4 is formed by sputtering on the ULSI 1, sputter particles enter the contact hole 5 not only in a vertical direction but also in other directions. Consequently, sputter particles are densely deposited on a shoulder portion of the contact hole 5 and this narrows the opening portion of the contact hole 5. As a result, a conductive film of a sufficient thickness cannot be formed on the inner surface of the contact hole 5. A film having only a thickness of about 0.1 micron is formed in the contact hole 5, and step coverage (i.e. the deposition ratio of the step portion) is 10% or less. Specifically, it is pointed out that when the conventional flat sputtering gun is used for an ULSI with a contact hole or via hole having a diameter of 0.5 micron or less, the step coverage and the deposition on the hole bottom decrease.
Furthermore, even with a magnetron sputtering apparatus capable of sputtering at a relatively low pressure, the generation of plasma is not stable unless the sputtering gas pressure is greater than 1 mTorr. Since the pressure in the entire space for processing must be increased up to a pressure ensuring stable processing, a considerable amount of sputter articles and sputter gas particles collide with one another. Thus, the conventional magnetron sputtering apparatus does not have sufficient sputtering efficiency.